[#45] Hook into the custom spatial previewer

A custom helper function checks if the format is one of the ones
supported by the external viewer maintained by GMU. The viewer URL and
the supported formats can be defined via config options.

ckanext.geodatagov.spatial_preview.url
ckanext.geodatagov.spatial_preview.formats

Support for a future default_srs parameter has been added.
Update production ini file

* Added plugin: waf_harvester_collection
* Removed plugins: spatial_query_widget wms_preview cswserver
* Added config options: ckanext.geodatagov.spatial_preview.url and
  ckanext.geodatagov.spatial_preview.formats (See #45)
* Removed config options: cswserver.*
Merge branch 'master' of github.com:okfn/ckanext-geodatagov